Null check added for city

src/engine/net/client/handlers/ItemProductionMsgHandler.java

@@ -393,6 +393,11 @@ public class ItemProductionMsgHandler extends AbstractClientMsgHandler {
         WorkOrder workOrder = ForgeManager.itemWorkOrderLookup.get(virtualItem);
 
         City city = workOrder.vendor.building.getCity();
+
+        if (city == null)
+            return;
+        ;
+
         city.transactionLock.writeLock().lock();
 
         try {